The trend toward integrating an ever-increasing number of communication standards into wireless communication devices such as cellular telephones (also commonly known as “mobile phones” or “handsets”) has created a need for integrated antennas operating in many distinct frequency bands. This need is exacerbated by a concurrent a trend toward highly integrated devices that are slimmer, smaller, and lighter than those in prior use. However, the design of the antenna is limited by fundamental relationships between size, bandwidth, and efficiency. Ability to use multiple antennas may also be limited due to the same constraints and also the potential for harmful coupling between the multiple antennas,
The design challenge of using a single antenna while maintaining small size and high efficiency is made even more difficult by the fact that the device is used in a variety of configurations and positions by users who manipulate the device and, in particular, the antenna, in ways that are difficult to predict. Moreover, even in a particular user position of the hand and/or head in proximity to the device, the antenna performance will vary greatly according to the location of the antenna within the device. While a nominal antenna provides an input impedance of 50 ohms, in actual usage the impedance at the antenna terminal can vary over a wide range, characterized by a voltage standing wave ratio (VSWR) of up to 10:1. It is a major challenge to maintain operation of the device with such a wide range of impedances, both in transmit and receive modes. In receive mode, the non-optimal source impedance may degrade noise figure, gain and dynamic range of the receiver. In transmit mode, the impedance mismatch may impact the efficiency, power gain, maximum output power and linearity of the transmit power amplifier. In the worst case, the high standing wave amplitude or possible oscillation caused by the mismatch in the circuit may damage the power amplifier.
One solution for mitigating these performance effects is to use a tunable impedance matching network to counteract the change in antenna impedance caused by the user interaction. Some tunable impedance matching networks have been implemented using MEMS switches, variable inductors (“varactors”), thin-film barium strontium titanate (BST) tunable capacitors, and silicon-on-sapphire (SOS) switches. Typically, these networks are placed between the antenna feed point and the radio-frequency transmitter and/or receiver circuits in the wireless device. This placement complicates the process of compensating for the user proximity to the antenna because it turns it into a two-dimensional problem requiring a more complicated matching network, such as with multiple adjustable components.
In addition to the tunable impedance matching network, implementations of such solutions may also require a way of determining that the antenna impedance (or, equivalently, the antenna resonant frequency) has changed. In some cases, this is accomplished by using a directional coupler and radio-frequency detectors. In some implementations, in addition to high complexity, another significant drawback with this approach is that it works during transmit mode and not during receive mode, which means that it cannot be used to mitigate the effects of user proximity to the antenna in important receive-only communication applications such as Global Positioning System (GPS).
Accordingly, benefits can be obtained by a system and method for mitigating the effects of user proximity on antenna performance that will work in both transmit and receive modes of operation and utilizes a simplified antenna impedance matching network.